Stacked microporous vapor permeable membrane distillation system

ABSTRACT

A distillation system incorporating a multiple stage still comprising microporous membranes in which heated and deaerated feed liquid is utilized, in part, as distilland, and in part, as a heating liquid for transferring heat to the distilland within the still to generate a thermal gradient and effect heat transfer required to produce distillate from the distilland. The feed liquid employed for heating is returned to a deaerator heater where it is combined with additional feed liquid to replace the distillate and effluent distilland liquid. Heat is transferred from the effluent and distillate to the additional feed liquid prior to introduction thereof into the deaerator heater.

United States Patent Rodgers [451 Mar. 21, 1972 [54] STACKED MICROPOROUSVAPOR PERMEABLE MEMBRANE DISTILLATION SYSTEM Franklin A. Rodgers,Brookline, Mass.

Assignee: Pactide Corporation, Cambridge, Mass.

Filed: Feb. 11, 1971 Appl. No.: 114,584

Inventor:

Continuation-impart of Ser. No. 102,811., Dec. 30, 1970, abandoned,which is a continuation-in-part of Ser. No. 838,872, July 3, 1969,abandoned, and a continuation-impart of Ser. No. 838,769, July 3, 1969,abandoned.

[56] References Cited UNITED STATES PATENTS 3 ,245,883 A 4719 66 Loebel"1.1263111 VAPOR IMPERMEABL MEMBRANES EFF UENT mum DISTILLATE /HOTDISTILLAND\ 3,340,186 9/1967 Weyl ..203/l0 X 3,388,045 6/ 1968 Goeldneret al. ...203/1 1 X 3,361,645 1/1968 Bobell ..202/l97 X 3,385,769 5/1968Brose ..202/l97 3,406,096 10/ l 968 Rodgers ..202/ l 72 PrimaryExaminer-Norman Yudkoff Assistant Examiner-David Edwards Attorney-Brownand Mikulka [57] ABSTRACT A distillation system incorporating a multiplestage still comprising microporous membranes in which heated anddeaerated feed liquid is utilized, in part, as distilland, and in part,as a heating liquid for transferring heat to the distilland within thestill to generate a thenna] gradient and effect heat transfer requiredto produce distillate from the distilland. The feed liquid employed forheating is returned to a deaerator heater where it is combined withadditional feed liquid to replace the distillate and effluent distillandliquid. Heat is transferred from the effluent and distillate to theadditional feed liquid prior to introduction thereof into the deaeratorheater.

15 Claims, 10 Drawing Figures Cool-ANT coouzn FEED- 1) as (MICROPOROUSVAPOR PERMEABLE MEMBRANE PATENTEDMAR21 I972 SHEET 2 BF 4 :IIIINVENTOR.

FRANKLIN A. RODGERS fit'mm m iATENTEDMRZ] I972 SHEET 3 [1F 4 INVENTOR.FRANKLIN A. RODGERS WilM ATTORNEW PATENTEUMARQI I972 SHEET [1F 4MPaJJFwE hZu: "Eu

m0 gl omwm Ow -O00 24 00 INVENTOR. FRANKLIN A. RODGERS m a/nd 7725Mamafi m ce STACKED MICROPOROUS VAPOR PERMEABLE MEMBRANE DISTILLATIONSYSTEM This application is a continuation-in-part of my copendingapplication Ser. No. 102,811 filed Dec. 30, 1970 and now abandoned as acontinuation-in-part of applications Ser. Nos. 838,872 and 838,769 bothfiled July 3, 1969 and now abandoned.

BACKGROUND OF THE INVENTION This invention relates to distillation and,more particularly, to a system and method of desalination utilizingselective heating and cooling of different portions of a multiple stagestill incorporating microporous membranes.

A number of U.S. patents and copending applications disclosedistillation systems and processes particularly adapted to thedemineralization, e.g., desalination, of water, in which a feed ordistilland liquid such as salt water is circulated in contact with oneside of a microporous membrane permeable to the vapor of the liquid andimpermeable to the liquid itself under the operating conditions. Adistillate liquid, e.g., distilled water, is maintained in contact withthe opposite side of the membrane. Heat is transferred to the distillandliquid to vaporize it while heat is transferred from the distillateliquid to cause the vapor of the liquid to pass through the membrane andbe condensed as distillate. Multiple stage distillation systems aredescribed in which the heat from the distillate of each stage istransferred to the distilland of a succeeding stage and so forth. Insystems of this type, several membranes or sections thereof are arrangedin stacked relation with alternate barriers which cooperate to formchannels on opposite sides of each membrane for distilland anddistillateliquids. Distillation methods of this type are described in applicant'sU.S. Pat. Nos. 3,406,096, 3,477,517, and 3,497,423; U.S. Pat. No.3,340,186; and in applicants copending application Ser. No. 524,366,filed Dec. 27, 1965.

A preferred embodiment of the distillation apparatus disclosed inapplicants aforementioned patents and application includes a multiplestage distillation unit in which both the microporous membranes and thespacers are formed of polymeric materials, the spacers being in the formof thin films. Suitable materials for the membranes are those which lendthemselves to the formation of microporous membranes having a highproportion of voids, e.g., 70 to 80 percent, and uniform poredistribution and which are either inherently poorly wettable ornon-wettable, e.g., hydrophobic, by the distillate liquid or can betreated to render them poorly wettable or non-wettable by the liquid.Examples of polymers particularly adapted to the formation of membranesuseful in water desalination are disclosed in the aforementioned patentsand application and include the fluorocarbons such aspolytetrailuoroethylene and polyvinylidene fluoride are preferredbecause they are inherently poorly wettable by water. Methods ofmanufacturing microporous membranes of this type are known in the artand include solvent-non-solvent systems such as disclosed for example inthe copending U.S. Pat. application of James L. Bailey et al., Ser. No.790,l92 filed Jan. 3, 1969, as a continuation-in-part of and replacementfor Ser. No. 577,593, filed June 16, 1966. Polymeric materials useful asthe barrier films are selected according to criteria includingcompatibility with the liquids involved, cost, ease of fabrication andassembly, useful operating temperatures and thermal conductivity.Polymers particularly suited for water desalination includepolycarbonates, polyesters, polyethylene, polypropylene and halogenatedpolyethylenes, particularly the fluorocarbons.

In a distillation unit of this type, the barrier films are corrugated toprovide channels for the flow of distillate and distilland liquids incontact with opposite sides of the porous mem branes which are securedto adjacent barrier films in selected regions particularly at the edgesof the membranes and/or barrier films to control the circulation of theliquids and confine the liquids to their proper channels. In a typicaldistillation system such as disclosed in the aforementioned patents andapplications, the membrane and barrier films are rectangular and stackedwith their edges in alignment and secured to one another adjacent theiredges to form a parallelepiped shaped unit. This unit comprising themembranes and films is coupled with external means for introducingliquids into and withdrawing liquids from the unit, means fortransferring heat to the unit and means for transferring heat from theunit. To conduct particular liquids to selected channels within theunit, aligned holes are provided through the stacked membranes and filmsand a selected channel is blocked in regions surrounding the alignedholes in the membrane and film defining said channel in order to preventthe flow of liquid into or from the selected channel through the holesin the membrane and film. The external means for introducing into andwithdrawing liquids from the unit as well as transferring heat to andfrom the unit generally comprise a pair of plates or headers betweenwhich the stack of membranes and films comprising the unit are engaged,together with liquid conduit means such as manifolds, coupled with theholes in the outer films of the unit for introducing and withdrawingliquids means for heating one of the headers and means for cooling theother header. Thus, a still of this type constructed according to theprior art comprises a unit formed of stacked membranes and filmsenclosed at least on two sides by heating and cooling elements as wellas liquid conducting means for coupling holes in the unit with externalaccessories such as circulating pumps, heat exchangers for recoveringheat from the product and effluent liquids, and liquid pretreatmentdevices for filtering, deaerating and chemically treating the feedliquid.

OBJECTS OF THE lNVENTlON It is an object of the present invention toprovide an improved desalination system including means for heating andcooling selected portions of a multiple stage still and utilizing heatedfeed liquid as both the distilland and the heating medium.

It is another object of the present invention to provide an improvedmethod of distillation which includes the steps of transferring heat toan incoming source liquid externally of the still and then utilizingthis heated liquid both as distilland and as a heat exchange liquid toachieve the requisite thermal gradient within the multiple stage still.

It is a further object of the present invention to provide an improveddesalination system and method of desalination which is more efficientin operation and more economical to construct and use.

Other objects of the invention will in part be obvious and will in partappear hereinafter.

SUMMARY OF THE INVENTION Briefly, the foregoing objects are achieved bya desalination system incorporating a multiple stage still includingmeans for heating and cooling selective portions of the still utilizingthe distilland liquid for heating the still.

It is the usual practice in distillation apparatus of the typedescribed, particularly in desalination apparatus utilizing source orfeed water having dissolved solids in addition to salts, to providemeans for preventing the feed liquid (water) to remove substances thatmight cause scaling, corrosion or the like, within the still itselfand/or within associated heat exchange devices such as heaters,deaerators, etc. It is also the practice to treat the heating liquid,that is, the liquid employed to heat the still so that there will be asubstantial investment in both the feed and heat exchange liquids interms of treatment prior to heating thereof so that a substantialsavings can be realized by utilizing the same liquid as both distillandand for heat transfer and heating this same liquid by the same means.External heating of the distilland is effected, in part, by transferringheat to the distilland from the distillate and the effluent. Heattransfer within the still is effected by circulating heated distillandthrough selected portions of the still while circulating a coolantliquid through other portions of the still. In this manner a favorablethermal gradient is established, preferably from the center of the stilloutward toward the exterior thereof and economies of operation arerealized in that the same treated, heated distilland liquid is utilizedboth as feed and as a heat transfer medium.

Additionally an improved method of desalination results because the heattransferred to incoming source water employed both as feed and as aheating medium, is conserved while the produce and effluent arecirculated through a heat exchanger and the heated source water cooledas a result of its use as a heat transfer medium, is reheated andrecirculated as both feed and the heat transfer medium. Consequently,the resulting system utilizes source water at ambient temperatures asthe input liquid while the product and effluent, at or near ambienttemperature, constitute the output liquids.

The invention accordingly comprises the apparatus possessing theconstruction, combination of elements and arrangement of parts, and themethod involving the several steps and the relation and order of one ormore of such steps with respect to each of the other which areexemplified in the following detailed disclosure, and the scope of theapplication of which will be indicated in the claims.

For a fuller understanding of the nature and objects of the invention,reference should be had to the following detailed description taken inconnection with the accompanying drawings wherein:

FIG. 1 is a somewhat schematic illustration of a distillation systemconstructed in accordance with the invention;

FIG. 2 is a perspective view of a still incorporated in the distillationillustrated in FIG. 1;

FIG. 3 is a perspective view of another embodiment of the still;

FIG. 4 is a perspective view of components of the stills of FIG. 2 and 3shown with parts broken away;

FIG. 5 is an exploded, perspective view of components of the still shownin FIG. 4, illustrating the method of fabrication and assembly;

FIG. 6 is an enlarged, elevational, sectional view of the still takensubstantially along the line 6-6 of FIG. 2;

FIG. 7 is a view similar to FIG. 5, the section being takensubstantially along the line 77 of FIG. 2;

FIG. 8 is an elevational view of a portion of the distillation systemillustrating the details thereof;

FIG. 9 is an elevational, sectional view of a portion of thedistillation system of FIG. 1, the section being taken substantiallyalong the line 99 of FIG. 1; and

FIG. 10 is an elevational, sectional view similar to FIG. 9,illustrating another embodiment.

Reference is now made to FIGS. 1 through 3 of the drawings wherein thereis illustrated a multiple stage distillation system and still embodyingthe invention. The distillation system shown in FIG. I includes a stillgenerally designated 10, including, as basic components thereof, amultiple stage distillation unit 12 composed of a multiplicity of porousmembranes and barrier films secured together in stacked relation andenclosed in a housing designated 14. The distillation unit 12 comprisesa multiplicity of sheetlike elements all substantially rectangular,having the same dimensions and arranged with their edges in alignment sothat unit 12 takes the form of a relatively thin parallelepipedon.

The components of unit 12 include a multiplicity of microporous, vaporpermeable membranes 16 and liquid and vapor impermeable barrier films18. The membranes and barrier films are arranged in face-to-facerelation in alternating order to form two rectangular stacks which arejoined together in a manner to be described to form unit 12. Eachmembrane 16 is preferably formed with a smooth or plane surface andanother surface having a corrugated appearance, being formed withalternating parallel depressions and raised sections having a generallysinusoidal configuration. As previously noted, each membrane is formedwith a high proportion, e.g., on the order of 80 percent, of microscopicthrough passages or pores for conducting the vapor of a liquid, such aswater, while preventing the passage of liquid by capillary action.Although polymeric materials useful for the membranes and methods ofmanufacture are disclosed in the aforementioned patents andapplications, particular mention should be made of polyvinylidenefluoride as a preferred membrane material and the so-calledsolvent-non-solvent" casting process as the preferred method of formingthe membrane, such processes having been described in US. Pat. Nos.1,421,341, 3,100,721, and 3,208,875 as well as the aforementioned Baileyet al. applications.

Although the construction and composition of barrier films 18 isdescribed in the aforementioned patents and application, it should benoted that the preferred polymeric material for the barrier films is apolycarbonate such as sold by General Electric Co. under the trademarkLexan." Films 18 are corrugated so that when interposed between adjacentporous membranes 16, as shown in FIGS. 6 and 7, the corrugations willcontact the surfaces of the porous membranes forming channels for theflow of liquids in contact with the membranes. As indicated in thedrawings, the corrugated barrier films l8 cooperate with porousmembranes 16 to form a multiplicity of distilland channels 20 eachbounded on one side by a microporous membrane and through which a feedor distilland liquid such as salt water is circulated; and amultiplicity of distillate channels 22 bounded by the opposite sides ofthe membranes in which vapor of the distilland liquid is condensed toform distillate liquid.

The essential components of a distillation stage of the still include amicroporous membrane 16 through which the vapor of the distilland istransferred and means such as barrier films l8 cooperating with themembrane to form distillate and distilland channels. For the purpose ofclarity of illustration, the thickness of the components have beenexaggerated and the still is shown in the drawings as including onlyfour distillation stages. However, it should be understood that inactual practice, such a still would normally comprise a very largenumber of distillation stages, for example, as many as 180. The actualnumber of stages, however, will depend upon the temperature differentialbetween the hottest and coldest stages and the temperature differentialbetween succeeding stages. In the multiple stage still, (see FIGS. 6 and7) heat is transferred to the distilland liquid in the distillandchannel (or channels) 20 of the first or hottest stage (or stages) tovaporize the distilland liquid. Heat is transferred from the distillateliquid of the last or coldest stage (or stages) of the still to condensethe vapor transferred through the adjacent membrane to form distillatein a distillate channel 22. In each stage, energy is transferred asflux, i.e., vapor, through the porous membrane and then transferred byconduction through the adjacent barrier film from the distillate to thedistilland liquid of the next succeeding stage. In the preferred formshown in the drawings, the still comprises two series of distillationstages with the first or hottest stages being located innermost andsuccessive cooler stages arranged outwardly therefrom. Thus,distillation unit 12 comprises two inner porous membranes 16 and,alternating outwardly therefrom, barrier films l8 and additional porousmembranes, four membranes and four barrier films being illustrated inthe drawings.

The distillate and distilland are formed and separated from one anotherto confine and conduct the flow of the liquids within the channels bydamming or blocking the channels in selected regions. The channels aredammed or blocked by sealing elements located within the channelsbetween adjacent membranes and barrier films and bonded to the facingsurfaces of the membranes and films. The distilland channels are locatedon the inner or hot side of each microporous membrane 16 and thedistillate channels are located on the opposite sides of the membranesfrom the distilland channels. The channels are blocked in such a waythat the flow of the distilland and distillate liquids is in paralleldirections from end-to-end of unit 12 and in the form shown, sealingelements 24 are bonded to the opposite lateral margins of adjacentmembranes and barrier films to prevent the admission or escape ofliquids from the distilland and distillate channels at the sidesthereof. The distillate channels 22 are blocked at opposite ends bysealing elements 26 and 28v secured between and to the end marginalsurfaces of adjacent membranes and barrier films between the cold sideof each membrane and the hot side of the adjacent barrier film. Thedistilland channels are blocked at one end, termed the exit end, bysealing elements 30 also bonded to the end marginal surfaces of adjacentmembranes and barrier films. The opposite ends of the distillandchannels remain open at the edges of the membranes to permit theintroduction of distilland liquid into the distilland channels.

Means are provided for introducing a liquid into or withdrawing a liquidfrom a particular channel at or near the end thereof at which thechannel is blocked. These means include outlets or conduits provided byholes formed in alignment through the stack of membranes, barrier filmsand sealing elements. Where the hole is formed in a sealing elementwithin a particular channel, there is no communication between thechannel and the hole because the channel is blocked in regionssurrounding the hole. However, where the channel remains unblocked inregions surrounding the hole, there will be communication between thehole and the channel. In this way, each group of aligned holes throughstacked membranes, films, and sealing elements forms a conduitcommunicating with selected channels.

Distillation unit 12 comprises two sets of membranes and barrier filmssecured together in face-to-face stacked relation by sealing elementsbonded to the membranes and barrier films. Each of these stacks, one ofwhich is shown in exploded form in FIG. 5, is bounded on one face by amicroporous membrane 16, on its opposite face by a barrier film 18 andincludes a multiplicity (one of each are shown) of barrier films andmembranes arranged in alternating order between the outer membrane andbarrier film. A pair of these stacks are arranged with membranes 16 inspaced face-to-face relation separated by corrugated spacer 32. Spacer32 is formed of a liquid and vapor impermeable polymeric sheet materialsuch as a polycarbonate and is corrugated so as to cooperate with theadjacent membranes 16 to form heating channels 34 through which aheating liquid is circulated for transferring heat to the still to helpestablish thermal gradients from the inside of the still outwardlytowards the exterior thereof. The thickness of the sheet materialcomprising spacer 32 may be substantially greater than the thickness ofthe sheet material comprising barrier films 18 and the corrugations inspacer 32 are substantially deeper, e.g., have a greater amplitude thanthe corrugations in barrier films 18, so that the flow capacity ofchannels 34 is substantially greater than the flow capacities ofchannels 20 and 22 defined by the corrugations in the barrier films. Thecorrugations of spacer 32 are parallel with the corrugations of thebarrier films, and spacer 32 is secured to the adjacent membranes l6 andchannels 34 are blocked at their lateral edges by sealing elements 36.By virtue of this construction, a heating liquid can be introduced intochannels 34 at one end edge of the stack and withdrawn from the channelsat the opposite end edge thereof after giving up heat to the liquidswithin the distillate and distilland channels during its passage throughchannels 34. In the preferred embodiment of the invention, the heatingliquid is the feed or distilland liquid, e.g., salt water, so that aportion of the heating liquid circulated through channels 34 istransferred as vapor through the microporous membranes l6 defining thesides of the heating channels.

Distillation unit 12 is enclosed within a housing 14 providing forcirculation of the various liquids including the feed liquid utilizedboth for heating as the distilland, and the cooling liquid. In the formshown in FIGS. 2, 3, 6 and 7, housing 14 comprises first and secondcomplementary sections 38 and 40, each having a generally rectangularmain wall 42 and dependent side walls 44 each formed with a flange 46.Housing sections 38 and 40 are secured to one another at flanges 46 toform a shallow chamber having length and width dimensions exceeding thelength and width of unit 12 and a depth dimension, measured between mainwalls 42, approximately equal to the dimensions of unit 12. Tofacilitate fabrication and assembly, housing sections 38 and 40 arepreferably identical in size, shape and conformation. Thus, only one setof tools is required and selective assembly is made unnecessary.

The corners of the housing are chamfered to form dependent comer wallsdesignated 48 disposed at 45 angles with respect to dependent side walls44 and including projecting sections 50 for receiving the corners ofunit 12 located within the chamber provided by housing 14. A sealantadapted to adhere to the housing sections and the edges of distillationunit 12 is introduced into projecting sections 50 to form a seal betweenthe corners of unit 12 and comer walls 48 of the housing to formchambers between the side walls of the housing and the edges of unit 12,these chambers being designated first, second, third, and fourth andnumbered 52, 54, 56, and 58, respectively. First chamber 52 is locatedat the edge of unit 12 at which distilland channels 22 are unblocked oropen while second chamber 4 is located at the opposite side of unit 12where all of the channels, except heating channels 34 are blocked. Thus,feed or distilland liquid introduced into first chamber 52 will enterand flow through distilland channels 20 and heating channels 34 towardthe opposite side of the unit. The portion of the heated distillandliquid which passes through the heating channels 34 and is cooled duringits passage, is collected within fourth chamber 54 from which it isconducted from the still.

First and second chambers 56 and 58 are located at the other two sidesof the still; that is, at the sides thereof at which all of thedistilland, distillate, and heating channels are blocked and incooperation with main walls 42, provide coolant channels designated 60through which a coolant liquid may be circulated in contact with outerbarrier films 18 of unit 12. Main walls 42 are formed with corrugationssimilar to those in spacer 32 and disposed at right angles to the latterso as to cooperate with outer barrier films 18 to form coolant channels60. The coolant channels are blocked at their sides to prevent flow ofliquid into and from third and fourth chambers 56 and 58 by sealingelements 62 bonded to the facing surfaces of outer barrier films 18 andmain walls 42 at the end margins of the outer barrier films. By virtueof this construction, a coolant liquid introduced into third chambers 56will be conducted through channels 60 against the outer surfaces of unit12 to fourth chamber 58 from which the coolant liquid may be withdrawn,thus establishing thermal gradients across sections of unit 12 betweenheating channels 34 and coolant channels 60.

Still 10 is a component of a distillation system illustrated somewhatschematically in FIG. 1 including means for supplying and circulatingheating, cooling, and distilland liquids to and through the still; meansfor treating the feed liquids; and means for transferring heat to andfrom the various liquids. The preferred system shown is speciallydesigned to produce distilled water from source water containingdissolved solids such as salts, and includes a conventional heatexchanger 64 into which a feed liquid such as pretreated saline waterfrom a suitable source is introduced at inlet 66. The hot product ordistillate, e.g., distilled water, and the hot effluent, e.g.,concentrated salt water, withdrawn from still 10 are introduced intoheat exchanger 64 at respectively inlets 68 and 70 are circulatedthrough the heat exchanger to transfer heat to the treated source waterwhich is withdrawn from the heat exchanger at outlet 72. The cooledproduct and effluent are discharged from the heat exchanger throughrespectively out lets 74 and 76. The pretreatment of the source liquid,for example salt water, may be conventional and performed in a knownmanner. It will usually include filtration to remove solid matter andchemical treatment to remove scale-producing agents such as sulfates andcarbonates.

In the distillation system illustrated in FIG. 1, the heated feed liquidfrom the heat exchanger is introduced into a deaerator heater 78 throughan inlet 80 where, in the case of water, the feed liquid is heated toits boiling point and noncondensable gases such as air are removed fromthe liquid. The heating means might take the form of electricalresistance heater, electrolytic heater, a gas or an oil fired heatexchange device depending on the availibility of a particular form ofenergy. Additionally, deaerator heater 78 may include control means suchas a thermostat for sensing the temperature of the incoming source waterand controlling the operation of the means for heating the incomingsource water.

Channels 34 have a depth or fiow capacity many times that of thedistilland channels so that the volume rate of How of the source liquidemployed primarily as heat exchange liquid and circulated throughchannels 34 to establish a thermal gradient, exceeds the combined volumerate of flow of the distilland liquid through the distilland channels ofseveral stages of the still. Thus, although the liquid transferred asvapor across the innermost porous membranes which bound each of channels34 across any other porous membrane 16 from the feed liquid circulatedthrough a distilland channel 20, the amount of liquid transferred issmall in proportion to the quantity of liquid circulated. Accordingly,the increase in concentration of dissolved solids, e.g., salt, in theliquid employed for heating is so small that the cooled heating liquid,withdrawn from the still, may be introduced together with incomingsource water and recirculated through deaerator heater 78. The heatedand deaerated feed liquid is withdrawn from the deaerator heater throughoutlet 82 and circulated by a pump 84 to and through the still wherein aportion of the heated feed liquid is circulated as distilland, whileanother portion thereof is circulated through channels 34 as the heatingliquid to help establish and maintain the requisite temperaturegradients within the still. During passage of the heating liquid throughchannels 34, vapor will pass from the liquid through innermost membranes16 to form distillate. The cooled heating liquid, as it exits from theheating channels, has a substantial value by virtue of both itspretreatment to remove unwanted constituents both mechanically, as byfiltering, and chemically to remove agents such as those which arecorrosive and/0r produce scale and heat content and accordingly,recirculated to the deaerator where it is introduced at inlet 86. Theincrease in salt concentration of the recirculated heating liquid isrelatively small and the quantity of the recirculated feed liquid issmall relative to the quantity of new or additional feed liquidintroduced in the deaerator heater, so that the salt concentration ofthe heated and deaerated feed water introduced into the still by way ofpump 84 will be only slightly greater than that of the source wateralone and will not effect the operation of the still. On the other hand,the utilization of the valuable heating liquid and the conservation ofthe heat energy by virtue of the recirculation of the heating liquidcoupled with the transfer of heat from the product and effluent t0 thefeed liquid will result in a substantial cost-savings and increaseoperating efficiency of the system.

in the operation of the still, the heated distilland liquid, e.g.,saline water, is caused to flow through the distilland channels betweenadjacent membranes and barrier films in a direction parallel with thedirection of the corrugations of the barrier films. The distillate orproduct is formed by condensation of vapor in the distillate channelsand in the preferred from of still shown, is withdrawn from the end ofthe still at which the distilland liquid is introduced so that flow ofdistillate is counter to flow of distilland. The distilland liquid isintroduced into the distilland channels at one end of unit 12 fromchamber 52 and flows through the distilland channels to conduits 88formed by aligned holes also designated 88 in the membranes l6, barrierfilms l8 and sealing elements bonded together by the sealing elementsincluding elements 26 and 62, as well as additional sealing elements 90located within heating channels 34 and bonded to the opposite surfacesof spacer 32 and the adjacent surfaces of membranes 16. Sealing elements90 are spaced from one another to permit flow ofthe heating liquidthrough the heating channels into second chamber 64 while blocking thechannels in regions surrounding conduits 88 thereby preventing flow ofthe heating liquid into conduits 88.

Similar conduits 92 are provided at the opposite end of the still forwithdrawing distillate liquid from the distillate channels 22. Conduits92 are constituted by the walls or holes, also designated 92, formed inthe membranes, barrier films, and sealing elements 62 and additionalsealing elements 94, similar to elements 90, provided within thedistilland channels 20 and heating channels 34. Elements 94 function toblock the channels in regions surrounding conduits 92 to prevent theflow of the feed liquid into conduits 92 through which the distillate iswithdrawn from the distillate channels. Conduits 88 and 92 are offsetfrom one another so that each of conduits 88 is aligned with a spacebetween a pair of conduits 92 and vice versa for reasons which willappear hereinafter. it will be noted that sealing elements 28 and 30,which block the ends, respectively, of the distillate and distillandchannels are formed with indentations such that conduits 88 and 92 openinto the distilland and distillate channels respectively. It will alsobe apparent that because of the spacing of the conduits formed throughthe membranes, barrier films and sealing elements, there is required tobe some flow of the distillate and distilland liquids transverse to thedirection of the corrugations in barrier films 18. To facilitate thislateral flow, the indentations in each of sealing elements 28 and 30 areseparated by projecting sections to form a dam having a sinusoidal edgeconfiguration tending to guide the flow of the liquid toward conduits 88and 92.

As previously noted, the surface of each porous membrane 16 defining oneside of each distilland channel is formed with alternating ridges andgrooves or corrugations, extending perpendicularly to the corrugationsof barrier films 18. This construction is provided to reduce thepossibility of blockages within the distilland channels due, forexample, to scaling and to facilitate the flow of the distillandtransverse to the barrier film corrugations in the regions of conduits88. The transverse corrugations or grooves in the facing surfaces whichbound each distilland channel 20 perform several functions tending toeliminate blockages due to scale buildup and facilitating lateral flow.First, the transverse corrugations tend to increase turbulence withinthe distilland liquid which in turn tends to prevent scale accumulationand adherence thereof to the membrane and barrier film surfaces. Shouldany scale buildup occur within a channel formed by a groove in a barrierfilm, the grooves in the adjacent membrane will provide alternate pathsor channels around the obstruction thereby eliminating regions ofstagnant liquid in which the concentration of the scale producing agentsmight tend to buildup and produce additional scale. The turbulent flowand alternate channels provided between the transverse grooves also tendto prevent particulate matter from becoming lodged in the distillandchannels and, in the event that a particle does become lodged in adistilland channel, provide alternate channels around the particle toprevent occurrence of stagnant regions tending to result in scalebuildup. Still another function of the grooves in the porous membranesis to provide channels for the transverse flow of the distilland liquidin the regions of conduits 88. This is particularly important in theseregions where the concentration of dissolved solids in and thetemperature of the distilland is greatest thus making conditions moreconducive to scale formation.

As previously noted, the sealing elements are preformed of an adhesivematerial adapted to bond to adjacent membranes, barrier films, the mainwalls of the housing and spacer 32. The sealing elements are preformedby casting a solution ofa thermosetting adhesive to form a sealingelement of the desired shape and thickness. Suitable sealing elementscan be formed by solvent casting such materials as an acrylo nitrilephenolic rubber base adhesives such as sold by B. F. Goodrich Companyunder the designation A864-B, on a silicone release paper and air dryingthe adhesive solution to form a layer having a thickness of the order ofl to 3 mils. The adhesive may either be cast in the desired shape or asa layer which, following drying, may be cut into sections of the desiredsize and shape.

The distillation unit 12 is then assembled by alternately positioningand stacking the membranes, barrier films, and sealing elements andspacer and then subjecting the stack of the assembled components to heatand pressure to activate and cure the adhesive thereby binding themembranes, films, and spacer to one another to form the variouschannels. Conduits 88 and 92 are then formed by the simple expedient ofdrilling through the unit, a conventional paper drill being suited forthis purpose.

Except for connections between conduits 88 and 92 and chambers 52, 54,56, and 58, the remainder of the assembly process involves locating unit12 between housing sections 38 and 40 together with sealing elements 62and subjecting the assembly, i.e., still 10, to heat and pressure tobond sealing elements 62 to main walls 42 of the housing. The flanges 46of the two housing sections are secured to one another either byconventional methods such as the use of an adhesive or by welding. Anadhesive or sealant in fluid form is introduced into projecting sections50 to form a seal between the corner walls of the housing and thecorners of unit 12 to divide the housing into chambers 52, 54, 56, and58. Sealants suitable for this purpose include room temperaturevulcanizing silicone rubbers having a low enough viscosity to permitintroduction into projecting sections 50 by way of a hollow needle orsyrmge.

As illustrated in FIGS. 3, 9, 10, each main wall 42 is formed atopposite ends with a row of circular, flat areas 96 each surrounded byan annular groove 98. The flat areas 96 are arranged such that each ofconduits 88 and 92 will underlie a fiat area 96 when the distillationunit 12 is assembled within housing 14. Thus, there will be twice asmany flat areas at each end of the still as there are conduits 88 and 92so that the housing sections can be reversed end for end or additionalconduits may be provided at either or both ends. To complete the still,holes are drilled through the flat area 96 so as to communicate withconduits 88 and 92.

A variety of means are provided for introducing liquids into andwithdrawing liquids from still 10, specifically for introducing heatedand deaerated feed liquid into first chamber 52, withdrawing the cooledand partially depleted heating liquid from second chamber 54,withdrawing the effluent liquid from distilland channels by way ofconduits 88, withdrawing distillate liquid from distillate channels 22by way of conduits 92, introducing coolant liquid into third chamber 56,and withdrawing the coolant liquid from fourth chamber 58. These meansare those structures which provide connections between external liquidconduits and main wall 42 of housing section 38 to couple the externalconduits with internal conduits 88 and 92 and chambers 52, 54, 56, and58. In the embodiment of still 10, illustrated in FIGS. 2 and 8, theseconnections are made by way of nipples or short tubes 102, 104, 106,108, 110 attached to the main wall of housing section 38. Nip ples 100,102, 104, 106, 108 are secured to main wall 42 surrounding the openingstherein for conducting the liquids into or from respectively chambers52, 54, 56, and 58. Nipples 108 are secured to forward wall 42surrounding holes 88 in the forward wall and nipples 110 are secured tothe forward wall around holes 92. Since the feed input through nipple100 is at a flow rate greater than the flow through any other nipple,the internal diameter thereof will be larger than the diameter ofnipples 108 and 110 which may be substantially the same in embodimentsin which the effluent flow rate approximates the product flow rate. Thenipples may be formed of the same material as the housing and may beattached to the forward wall 42 in any conventional manner such as bywelding or by a suitable adhesive. The nipples are connected to externalconduits by conventional means such as elbows 112 formed of an elastomeror flexible polymer.

In the embodiment of the still shown in FIG. 3, the connections for thefeed and cooled feed liquid, and coolant liquid are made by way ofnipples 100, 102, 104, and 106 as described above. However, means areprovided for making direct connection with conduits 88 and 92 withoutthe necessity for attaching nipples 108, to the forward walls. Thisconstruction has the advantage of making the connecting means apermanent part of the distillation system and facilitates replacement ofstill 10, the component of the entire system most likely to requirereplacement. These means for making connections to the still to providefor removal of effluent and product liquid are illustrated in FIGS. 1,3, 9, and 10 and include blocks 114, 116 adapted to be clamped againstmain wall 42 overlying walls 88 and 92 respectively. Each of the blocksis formed with a plurality of bores 118 adapted to be aligned with holes88 and 92 and a tube or nipple 120 is engaged in each bore 118 andprojects therefrom so as to couple with an external conduit by way of anelbow 112.

Two alternate systems are shown for making a connection between a tube120 and a portion of the forward wall comprising annular groove 98surrounding each of holes 88 and 92. In the form shown in FIG. 8, thesemeans comprise a tube 122 formed of an elastomer and engaged in acounterbore 124 in the block (114 or 116). Tube 122 projects beyond thesurface of the block facing the main wall 42, and is held inliquid-tight engagement with theforward wall within annular groove 98when the block is pressed against the main wall. In the embodiment shownin FIG. 9, a conventional O-ring 126 is provided in a counterbore orannular groove 128 surrounding bore 118 (in block 114 or 116) and isadapted to be pressed into sealing engagement with annular groove 98 inmain wall 42 when the block is pressed against the main wall. Thislatter structure has the advantage of simplicity and the fact that theseal becomes more effective when the pressure is increased. Thus byvirtue of this construction, all of the connections between the stilland the external conduits for withdrawing effluent and product liquidsfrom the still can be made simply by clamping blocks 118 against thestill.

In the novel system of the invention, the treated, heated and deaeratedsource liquid performs dual functions. A portion is circulated throughthe still for the purpose of promoting heat transfer within the stilland another portion of the heated and deaerated source liquid iscirculated as distilland. A coolant liquid such as untreated sourceliquid can be circulated through outer portions of the still toestablish a thermal gradient, the center of the still to the outerportions thereof thereby, required for efficient still operation.Finally, the product liquid and effluent are circulated through a heatexchanger for purposes of transferring heat to incoming source liquid.The treated source liquid circulated through the still for purposes ofpromoting heat transfer represents a valuable material by virtue of itstreatment and heat content and is recirculated through the deaerator incombination with incoming treated and preheated source liquid forcirculation through the still.

It should be apparent that the system of the present invention providesfor improved efficiency of distillation of salt water to producedistilled water as a result of utilization of the treated, deaeratedsource liquid for heating together with vaporization thereof takingplace to produce product liquid at the first stage (or stages) of thestill, and then recirculating the cooled liquid through thedeaerator-heater in conjunction with fresh incoming source liquid.Accordingly, this recirculated source liquid needs less heat to bring itup to temperature, its having given up very little heat in its firstcirculation, and additionally, less heat is required to effectdeaeration since the recirculated water is already deaerated.Additionally, the remaining heat content of the product withdrawn fromthe still is, in turn, transferred to incoming source liquid in the heatexchanger. Likewise the effluent which is the residue of source liquidthat has had liquid extracted therefrom, contains heat which is alsotransferred to incoming source liquid by the heat exchanger.Consequently, a high degree of efficiency is realized both in the systemand in the utilization of the system to perform the novel distillationdescribed herein.

Since certain changes may be made in the above apparatus and methodwithout departing from the scope of the invention herein involved, it isintended that all matter contained in the above description or shown inthe accompanying drawings shall be interpreted as illustrative and notin a limiting sense.

What is claimed is:

l. In a distillation system comprising a still including multiple stageseach including a microporous vapor permeable membrane through whichvapor of a distilland liquid in contact with one side of said membraneis transferred through said membrane and condensed in contact with theopposite side thereof to form distillate, the improvement comprising, incombination:

said multiple stage still including means for receiving the heated feedliquid and conducting a portion of said feed liquid through stages ofsaid still as distilland in contact with said membranes, means forconducting another portion of said feed liquid through a first sectionof said still for transferring heat to said distilland to vaporize thelatter, means for conducting effluent from said still, said effluentconsisting of distilland from which vapor has been transferred, andmeans for separately conducting said other portion of said feed liquidfrom said still following transfer of heat therefrom;

liquid treatment means including means for heating said feed liquid;

means for circulating said feed liquid through said liquid treatmentmeans and introducing said feed liquid into said means within said stillfor receiving said feed liquid; and

means for introducing said other portion of said feed liquid withdrawnfrom said still into said liquid treatment means in combination withadditional liquid from said source.

2. A distillation system as defined in claim 1 wherein said liquidtreatment means include a deaerator heater.

3. A distillation system as defined in claim 1 wherein said stillincludes means for circulating a coolant liquid through a second sectionof said still for transferring heat from said distillate.

4. A distillation system as defined in claim 3 wherein said firstsection of said still includes at least a passage between said membranesnear the center of said still and said second section includes passageslocated near the exterior of said still for establishing a thermalgradient across said membranes outwardly from said first section.

5. A distillation system as defined in claim 4 wherein said stillincludes a distilland passage adjacent each of said membranes forconducting said feed liquid in contact with said each membrane and saidfirst section of said still has a flow capacity substantially exceedingthe combined flow capacity ofa plurality of said distilland passages.

6. A distillation system as defined in claim 1 wherein said stillcomprises a multiplicity of said membranes and impermeable filmsarranged in alternating, stacked relation outwardly from a pair of saidmembranes and said first section of said still comprises at least aheating channel bounded on opposite sides by said pair of saidmembranes.

7. A distillation system as defined in claim 6 wherein each of saidfilms cooperates with one of said membranes to form a distilland channelfor conducting said distilland in contact with said membrane and theflow capacity of said heating channel is greater than the combined flowcapacity of a plurality of said distilland channels.

8. A distillation system as defined in claim 6 wherein said stillincludes coolant passages each bounded on one side by the outerimpermeable films of said still, said system including means forcirculating a coolant liquid through said coolant channels to establishthermal gradients across said stacked membranes and said impermeablefilms outwardly from said first section of said still.

9. A distillation system as defined in claim 1 including heat exchangemeans for transferring heat between liquids;

means for circulating liquid from a source through said heat exchangemeans to said liquid treatment means; means for separately circulatingsaid effluent and said dlstillate liquids through said heat exchangemeans to transfer heat from said effluent and distillate liquids to saidliquid from said source.

10. In a method of distillation in which liquid supplied from a sourceis subjected to treatment including heating and deaeration in adeaerator heater and is circulated as feed liquid at a relatively lowvolume rate of flow through each of a multiplicity of distillandchannels each bounded on one side by a microporous membrane to transfervapor from said feed liquid through said membrane to one ofamultiplicity of distillate channels bounded on one side by the otherside of said each membrane, the vapor of said feed liquid is condensedto form distillate liquid in said distillate channels, and heat istransferred from distillate in each of said distilland channels to thefeed liquid in a succeeding distilland channel to vaporize the latter,the improvement comprising, in combination:

transferring to the environment heat from distillate in the last ofasuccession ofsaid distillate channels; and

circulating a portion of said liquid from said deaerator at a relativelyhigh volume rate of flow through a heating channel defined at least onone side by one of said membranes to establish a temperature gradientacross a succession of said distillate and distilland channels, arrangedat alternating order, from said heating channel to said last distillatechannel.

11. A distillation method as defined in claim 10 including the steps ofwithdrawing said liquid from said heating channel following circulationof said liquid therethrough and recirculating said liquid withdrawn fromsaid heating channel together with additional liquid from said sourcethrough said deaerator to heat both of said liquids and deaerate saidadditional liquid.

12. A distillation method as defined in claim 11 including the steps ofwithdrawing said feed liquid and said distillate liquid from saiddistilland and distillate channels, respectively, and circulatingseparately through a heat exchanger, said feed and distillate liquidswithdrawn from said distilland and distillate channels and saidadditional liquid from said source to transfer heat from said feed anddistillate liquids to said additional liquid prior to circulating thelatter through said heat exchanger.

13. A distillation system as defined in claim 10 wherein vapor istransferred from said liquid during circulation thereof through saidheating channel, across said membrane defining a side of said heatingchannel to a distillate channel and is condensed in the latter to formdistillate.

14. A distillation system as defined in claim 10 wherein heat istransferred to the environment from a plurality of said distillatechannels each of which is the last of a succession of said distillatechannels arranged outwardly in alternating order with distillandchannels from said heating channel to establish and maintain temperaturegradients across said successions of said distillate and distillandchannels.

15. A distillation system as defined in claim 10 wherein said liquidfrom said source is water containing dissolved solids.

2. A distillation system as defined in claim 1 wherein said liquidtreatment means include a deaerator heater.
 3. A distillation system asdefined in claim 1 wherein said still includes means for circulating acoolant liquid through a second section of said still for transferringheat from said distillate.
 4. A distillation system as defined in claim3 wherein said first section of said still includes at least a passagebetween said membranes near the center of said still and said secondsection includes passages located near the exterior of said still forestablishing a thermal gradient across said membranes outwardly fromsaid first section.
 5. A distillation system as defined in claim 4wherein said still includes a distilland passage adjacent each of saidmembranes for conducting said feed liquid in contact with said eachmembrane and said first section of said still has a flow capacitysubstantially exceeding the combined flow capacity of a plurality ofsaid distilland passages.
 6. A distillation system as defined in claim 1wherein said still comprises a multiplicity of said membranes andimpermeable films arranged in alternating, stacked relation outwardlyfrom a pair of said membranes and said first section of said stillcomprises at least a heating channel bounded on opposite sides by saidpair of said membranes.
 7. A distillation system as defined in claim 6wherein each of said films cooperates with one of said membranes to forma distilland channel for conducting said distilland in contact with saidmembrane and the flow capacity of said heating channel is greater thanthe combined flow capacity of a plurality of said distilland channels.8. A distillation system as defined in claim 6 wherein said stillincludes coolant passages each bounded on one side by the outerimpermeable films of said still, said system including means forcirculating a coolant liquid through said coolant channels to establishthermal gradients across said Stacked membranes and said impermeablefilms outwardly from said first section of said still.
 9. A distillationsystem as defined in claim 1 including heat exchange means fortransferring heat between liquids; means for circulating liquid from asource through said heat exchange means to said liquid treatment means;means for separately circulating said effluent and said distillateliquids through said heat exchange means to transfer heat from saideffluent and distillate liquids to said liquid from said source.
 10. Ina method of distillation in which liquid supplied from a source issubjected to treatment including heating and deaeration in a deaeratorheater and is circulated as feed liquid at a relatively low volume rateof flow through each of a multiplicity of distilland channels eachbounded on one side by a microporous membrane to transfer vapor fromsaid feed liquid through said membrane to one of a multiplicity ofdistillate channels bounded on one side by the other side of said eachmembrane, the vapor of said feed liquid is condensed to form distillateliquid in said distillate channels, and heat is transferred fromdistillate in each of said distilland channels to the feed liquid in asucceeding distilland channel to vaporize the latter, the improvementcomprising, in combination: transferring to the environment heat fromdistillate in the last of a succession of said distillate channels; andcirculating a portion of said liquid from said deaerator at a relativelyhigh volume rate of flow through a heating channel defined at least onone side by one of said membranes to establish a temperature gradientacross a succession of said distillate and distilland channels, arrangedat alternating order, from said heating channel to said last distillatechannel.
 11. A distillation method as defined in claim 10 including thesteps of withdrawing said liquid from said heating channel followingcirculation of said liquid therethrough and recirculating said liquidwithdrawn from said heating channel together with additional liquid fromsaid source through said deaerator to heat both of said liquids anddeaerate said additional liquid.
 12. A distillation method as defined inclaim 11 including the steps of withdrawing said feed liquid and saiddistillate liquid from said distilland and distillate channels,respectively, and circulating separately through a heat exchanger, saidfeed and distillate liquids withdrawn from said distilland anddistillate channels and said additional liquid from said source totransfer heat from said feed and distillate liquids to said additionalliquid prior to circulating the latter through said heat exchanger. 13.A distillation system as defined in claim 10 wherein vapor istransferred from said liquid during circulation thereof through saidheating channel, across said membrane defining a side of said heatingchannel to a distillate channel and is condensed in the latter to formdistillate.
 14. A distillation system as defined in claim 10 whereinheat is transferred to the environment from a plurality of saiddistillate channels each of which is the last of a succession of saiddistillate channels arranged outwardly in alternating order withdistilland channels from said heating channel to establish and maintaintemperature gradients across said successions of said distillate anddistilland channels.
 15. A distillation system as defined in claim 10wherein said liquid from said source is water containing dissolvedsolids.